The approach phase, that is to say, the period immediately preceding the landing, takes place in airport zones where traffic is dense. There are so-called precision approaches commonly called instrument approaches, or “low RNP” (RNP standing for “Required Navigation Performance”) type approaches. By allocating a restricted air space to the aircraft, these approaches provide a solution to the significant increase in air traffic and make it possible to reduce the minimum landing decision thresholds (Minimum Decision Altitude MDA), to calculate curved approach flight plans minimizing flights over inhabited areas, to find new approach or departure paths in mountainous environments. To move in a space defined by an instrument approach, an aircraft must have sufficient performance levels to ensure the safety of the aircraft in that space. However, if the aircraft has a failure, for example, an engine failure, which means that it can no longer ensure the level of precision required in a precision approach or else for reasons of momentary or prolonged unavailability of the planned landing runway, the pilot may have to divert the aeroplane from an initial flight plan, in which case he must follow an interrupted approach procedure commonly referred to by the expression “missed approach” that will hereinafter be referred to as escape procedure. This procedure makes it possible to evacuate the runway or the space allocated to the trajectory of the aircraft by following a secured, so-called escape trajectory on which there is no risk of collision with another aircraft or with the reliefs in the airport area. In this procedure, the aircraft goes around to gain altitude.
A flight procedure corresponds to a flight plan, that the aircraft is assumed to follow between an initial position and a final position. A flight plan is a detailed description of the trajectory that the aircraft is assumed to follow. The trajectory includes a lateral trajectory which is generally characterized by a chronological sequence of segments linking pairs of waypoints described by their position in the horizontal plane and arcs of circle, both to handle the heading transitions between segments at the waypoints and to follow certain curved segments. The trajectory also includes a vertical trajectory, a trajectory in the vertical plane. The waypoints are characterized by their time of passage.
The aircraft are conventionally equipped with a flight management system, hereinafter referred to as FMS. The FMS is responsible for the design of the flight plans, the construction of the lateral trajectory and of the vertical trajectory. The vertical trajectory is obtained by the integration in the vertical plane of the position of the aircraft along this lateral trajectory in order to obtain predictions at the waypoints (altitude, speed, time, fuel predictions). The integration of a model of the aircraft is made possible by the provision by the aircraft manufacturer of the aerodynamic and motive parameters of the craft, stored in a “performance database” in the FMS system and guidance setpoints adapted to follow the flight plan. Currently, when a failure occurs, the FMS presents to the pilot, via a human-machine interface, one or more possible flight procedures that the aircraft could follow in the continuation of the flight. It is for the crew to choose the procedure that the aircraft will follow thereafter in light of the capabilities of the craft.
In addition to the escape procedures, the FMS may be required to present to the pilot omnidirectional departure procedures. In these procedures, the lateral trajectory to be followed is defined, not by a succession of points, but by one or more directions to be followed in the horizontal plane and, possibly, by the transition curves between two successive directions. A direction to be followed in the horizontal plane is commonly called a heading. An omnidirectional departure procedure is a procedure during which the aircraft is diverted from an initial flight plan from an initial position to an arrival position by following a first heading up to a given altitude and then a second heading up to the final position. In some airports, a departure authorization may include standardized instrument departure instructions, commonly called SID (Standard Instrument Departure). A standard instrument departure SID is a planned departure procedure originating from an air traffic control ATC authority, published in graphic and text form and intended for the pilots and the controllers. The SIDs handle the transition from the take-off position to a flight plan. The SIDs conventionally include two successive headings in the horizontal plane.
There are many constraints that affect the pilot, who must take into account a large volume of information before taking the decision to follow a flight procedure. The pilot must be able to select, by himself, with total awareness, a flight procedure that is assumed to have to be followed by the aircraft after the selection, that ensures the safety of the passengers. In order for the pilot to compare the capabilities of the aircraft with the capabilities required on a flight procedure, the FMS calculates, for the flight procedures that it presents to the pilot, a theoretical climb slope. The theoretical climb slope is the mean slope with which an aircraft must be capable of flying (in other words, the mean slope that the aircraft must be capable of flying) to ensure the safety of the passengers in the procedure. The term “slope” should be understood to mean the inclination of the aircraft, or of the trajectory followed by the aircraft, relative to the horizontal plane.
The pilot calculates a slope, called flyable slope or climb capability of the aircraft, with which the aircraft is able to move by means of tables grouped together in technical documentation. Each table links the values of the slope with which an aircraft is able to move with the various flight parameters such as the weight of the aircraft, the temperature, the altitude, the state of operation of the engines. The pilot then compares the flyable slope with the theoretical climb slope. He then chooses a flight procedure on which the theoretical climb slope is less than or equal to the flyable slope.
Calculating the slope that can be flown by the aircraft is a lengthy and tedious task. Searching through tables in the paper documentation takes time. It takes that much more time when the pilot wants to calculate a precise slope taking into account a maximum of parameters. He must look up a plurality of tables to calculate a precise slope according to the values of a plurality of flight parameters. Thus, the calculation of the flyable slope is imprecise if the pilot has only a limited time to perform his calculations. It is also unreliable, because the pilot may easily make calculation errors. Since the pilot uses the slope calculation as the basis for choosing a flight procedure, there is then no guarantee that the aircraft can follow the chosen procedure with the safety level required notably with regard to the relief. Having the task of calculating the flyable slope taken over by the onboard personnel is also not without risks. In critical stages of the flight, such as the approach phase or climb phase following take-off, the onboard personnel are already heavily stressed. The stress level is maximal because of the manoeuvres associated with the take-off or the go-around. It is also in these areas that the systems are most likely to raise alerts and inopportunely monopolize the attention of the piloting personnel.